Many fermentation processes have been used to convert biomass to various fuels. Currently, the fermentation of syngas, which is a mixture of CO, H2 and N2, is being studied for the production of ethanol. In our studies, an anaerobic microbial catalyst Clostridium carboxidivorans was used and the key performance parameters, such as growth rate and ethanol/acetic acid production rates, were compared with regards to redox potential and sulfide concentration. Studies were carried out in both 100 ml bottles with daily replacement of headspace gas as well as in 1-liter bioreactors with continuous gas flow. During our studies, the reducing agent cysteine-sulfide was applied to maintain the anaerobic environment.

In all studies, microbial catalyst was observed to first produce acetic acid as a fermentative product while in the growth phase. As expected, a switch was observed from acetic acid to ethanol as the cell concentration reached steady state. Furthermore, it was observed that the acetic acid decreased, indicating that the cells could be converting the acid to ethanol. Interestingly, we observed that both the cell growth and the switch between acetic acid production and ethanol production corresponded with changes in the redox potential. Different redox potentials were observed at different growing or production rates. These observations indicated that redox potential may play important roles during the fermentation process. In order to improve cell growth and ethanol production, a model of redox control was built and experiments were conducted at different levels of redox potential.

In addition, we found sulfide was not only scavenging the residual oxygen, but also affecting the fermentative performance. Since there was an equilibrium of HS- and H2S between gas and liquid phases, the volatile H2S could leave the system with the continuous gas flow in reactor studies or with outlet gas during the headspace gas replacement in bottle studies. The sulfide loss with continuous gas flow in the reactors was much faster than that in the bottles. The different sulfide concentrations may cause different redox levels, and may affect the cell growth and ethanol production. Models of sulfide loss in both reactors and bottles agreed with experimental data. Based on the models, studies at different sulfide levels were conducted and the optimum sulfide concentrations for cells growth and ethanol production were found.